Lecture 13 Global Wind Patterns and the Oceans EOM Global Wind Patterns and the Oceans Drag from wind exerts a force called wind stress on the ocean surface in the direction of the wind The currents in the upper ocean are driven by the wind In general they tend to flow in a similar direction to the prevailing wind Poleward currents are warm equatorward currents cold so ocean currents move heat from the tropics toward the poles doing a comparable amount of heat transport as occurs in the atmosphere This heat transport helps warm the climate of higher latitudes while cooling the tropics In the winter strong temperature contrasts occur between the warm Gulf Stream and the cold interior of N America This produces a strong jet stream and a storm track region of particularly intense storms along the Atlantic coast of N America Current Sea Surface Temperatures Unisys WWW site Upwelling Along the Pacific coast in the summer ocean temperatures stay quite cold particularly off N California This is due to upwelling of cold and nutrient rich water induced by the prevailing N winds EOM How Alongshore Winds Make Upwelling EOM Wind stress pushes surface water southward Coriolis force deflects surface water to the right offshore Cold subsurface water upwells to take its place Fog or persistent low cloud often forms in the chilled overlying air Equatorial upwelling Well below the ocean surface it is icy cold even in the tropics A thermocline separates the warm surface waters from the cold deep waters Prevailing tropical easterly winds tend to pile up the warm surface waters on the west side of ocean basins so the equatorial W Pacific thermocline is deep 200 m while the E Pacific thermocline is shallow x s denote TAO buoy ocean temperature measurements TAO WWW site Easterly winds also induce equatorial upwelling since Current NH Wind stress CF Wind stress CF Current SH the easterly winds push on the ocean surface surface currents adjust so Coriolis force on the water balances wind stress In the northern hemisphere the resulting current is northward In the southern hemisphere the resulting current is southward To replace the diverging surface water cold water is upwelled at equator This lowers SST along the equator If winds change to westerly there is equatorial downwelling and no source of cold water so SST remains warm even near equator If the thermocline is shallow E Pacific upwelling easily cools SST El Nino Southern Oscillation ENSO A coupled tropical atmosphere ocean phenomenon Normal conditions Warmest water low surface pressure and persistent T storms in W Pacific Strong subtropical highs in E Pacific Easterly winds and cool upwelled water along the equator in E Pacific Southerly prevailing winds off TAO WWW site the Peruvian coast produce cold upwelling good fishing La Nina is an intensification of the east winds upwelling and equatorial cooling Every few years an El Nino or warm event Central equatorial Pacific ocean warms Storms and low surface pressure move to central and east Pacific the Southern Oscillation Trade winds and coastal southerly winds weaken Off Peru upwelling diminishes and ocean surface warms an El Nino event leading to fish dieoff TAO WWW site and ocean warming The changed tropical Pacific wind and pressure patterns influences the entire tropics and much of the midlatitudes The mega El Nino of 1997 1998 Ocean 3 5 C warmer than normal along equator in E Pacific off Peru Unisys WWW site The La Nina of 1998 1999 Ocean 2 C colder than normal along equator in central Pacific ENSO over the last 20 years Nino 3 SST anomaly 2S 2N 90 150 W El Nino La Nina Southern Oscillation Tahiti Darwin pressure During El Nino Tahiti to Darwin PGF is weak killing and east winds and upwelling along the equator During La Nina T D PGF is strong so east winds and upwelling are strong along the equator promoting cold SST The ENSO Ocean Atmosphere Feedback ocean temperature pattern surface pressure pattern storms winds ocean currents and upwelling pattern With monitoring of the temperature in the upper ocean in the equatorial Pacific and the atmosphere using NOAA s TAO Tropical Atmosphere Ocean buoy array currently run out of PMEL here in Seattle and satellite and other measurements we can now forecast an El Nino with considerable skill up to a year in advance